1. Field of the Invention
The present invention relates to a rotational position sensor. More particularly, it relates to an inductance type rotational position sensor in which a rotational position of a rotational position-detected member is detected in accordance with variations in inductance of coils.
2. Description of the Related Art
Among rotational position sensors for detecting a rotational position of a motor and the like, there is an inductance type rotational position sensor for detecting rotational position in accordance with variations in inductance of coils. The inductance type rotational position sensor is provided with: a sensing side coil, which extends around the outermost circumference of a core and is displaceable along the core to induce a magnetic field having a high frequency; and, a movable short ring, which is disposed to permit movement thereof relative to the core.
In the rotational position sensor of this type, the core has a ring shape, which is concentric with a revolving shaft of the motor. The movable short ring is connected to the revolving shaft of the motor so as to turn jointly therewith. To be specific, the rotational position sensor is designed such that an interrupted position of a magnetic flux generated in the sensing side coil varies with a shifting position of the movable short ring turning jointly with the revolving shaft of the motor, thereby varying an inductance of the sensing side coil. In addition, in the same type of the rotational position sensor, a correcting side coil, i.e., a referencing side coil is disposed next to the sensing side coil, while a fixed short ring is anchored to the core in place where the correcting side coil lies. The correcting side coil and the fixed short ring serve to correct variations in the inductance of the sensing side coil influenced by temperature and the like.
Accordingly, when the inductance of the sensing side coil is compared with that of the correcting side coil, a rotational position of the revolving shaft of the motor can be detected on the basis of the resulting inductance value.
In conventional inductance type rotational position sensors of this type, spaces for screws and the like to be incorporated therein are essential before the fixed short ring is anchored to the core or the core itself is fixed. As previously described, the sensing side coil and the correcting side coil are provided around the core; and, the core is simply in the form of a ring that is concentric with the revolving shaft of the motor. A shiftable range of the movable short ring is thereby limited to a great degree. To be specific, the shiftable range of the movable short ring is limited when the spaces for fixing screws and the like are provided. This results in an inconvenience in which the rotational position of the revolving shaft of the motor is difficult to be detected over a wide range, i.e., over a wide angle range. A detectable scope of the rotational position sensor is thus reduced.
Furthermore, in the conventional inductance type rotational position sensors of this nature, different windings of the sensing side coil and the correcting side coil are coiled "in an exposed state" or "an impregnated state" before simply soldered to terminals. This increases the likelihood that the different windings are influenced by vibrations, jarring in the course of assembling work, or variations in ambient temperature. Therefore, improvements in reliability or durability must be achieved.
A high degree of reliability is required for the rotational position sensor. In the above-described rotational position sensor, the core preferably has a low level of loss because the rotational position sensor is designed to apply a high frequency to the referencing side coil and the sensing side coil, thereby detecting variations in the resulting magnetic fields. For this reason, the usual practice is to laminate thin plates such as silicic steel plates or permalloy so as to form an iron core.
However, in the core formed by laminated thin plates, a thin plate forming a surface of the core may be deformed or detached therefrom by vibration or the like. This causes an inconvenience in which the thin plate comes into contact with a turning movable short ring, thereby decreasing reliability of the rotational position sensor.
Sensing accuracy of the rotational position sensor depends largely on fine adjustment for successfully assembling the rotational position sensor. To be specific, an initial position of the movable short ring is shifted when a relative position between the rotational position sensor and an assembled component, such as the motor or the like, is out of line with a given position at which these are assembled. An initial value for an output voltage of the rotational position sensor (or, a graphical zero point illustrating sensor characteristics) in relation to the same rotational position is thereby shifted. In addition, in the rotational position sensor itself, a magnetic path for the referencing side coil varies when the fixed short ring on the core in relation to the referencing side coil assumes an out-of-line position of the fixed short ring. A variation ratio of the output voltage to the rotational position can be graphically depicted to illustrate sensor characteristics. As a result, problems arise in which characteristics of the rotational position sensor vary or sensing accuracy thereof decreases.